The use of robotic positioning systems becomes increasingly widespread in the industry. In general a robotic system comprises a base portion and a plurality of interconnected arm members mounted thereon. A mechanical hand is carried at a distal end of the arm assembly to grip a functional member such as a welding torch or spray gun. The interconnected arm members are moved by respective actuators or motors under command signals of a control unit so that the functional member may reach a desired spacial position within the limits of these arm movements.
While such conventional robotic systems are fairly satisfactory to meet usual positioning needs, it has been found that they entail serious problems when assembled in a field of use where a high degree of positioning precision is essential. Difficulties especially arise in an electrical machining system or center in which an ultra-high precision is critical not only as to the three-dimentional position of a reference point but as to a particular direction in which the tool electrode must be oriented or must remain oriented. In rotating each arm in the robotic system, an error is unavoidably created with respect to a target angular position. The error is partly caused by a deformation or deflection of the arm and increases as the arm length increases. Such errors become cumulative through the interconnected arms towards the distal functional end where the mechanical hand carries the tool electrode. An error or deviation of movement of a portion of a member from a movement command can be corrected through a usual sensor and feedback means as has been done in the art. It has been found, however, that such corrections do not correct but may even give rise to an error in the direction in which that member or the eventual functioned member must be oriented.